It joins another for a breather (7)
I believe the answer is:
nostril
'a breather' is the definition.
(I've seen this before)
'it joins another' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'another' could be 'r' (I've seen this in other clues) and 'r' is found in the answer.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for nostril that I've seen before include "Part of nose" , "A hole in the head" , "Breathing space" , "Nasal opening" , "Breathing aperture" .)